Silicon carbide has superior properties regarding high-power, high-frequency, and high-temperature electronics, when compared to today's commercially available semiconductors. The main advantage of using SiC over other semiconductors is its high bandgap of ˜3 eV, more than double that of Si or GaAs. Additionally SiC has a high thermal conductivity. The high bandgap allows the device to operate at increased temperatures without electron excitation from the valence to conduction band. Also, the combination of high bandgap and high breakdown field allows for extremely high power densities. SiC is physically rugged and chemically inert, which is an advantage when operating in harsh environments.
To take advantage of the properties of SiC, high-quality large-area boules with low defect densities must be produced economically. To date most bulk SiC is grown using seeded sublimation techniques, also known as modified Lely methods. The technique was first introduced by J. A. Lely, U.S. Pat. No. 2,854,364, and has been improved upon throughout the years as shown by Davis et al, U.S. Pat. No. 4,866,005. Boules with diameters up to 100 mm grown employing this technique are commercially available. For this process a graphite crucible is typically heated to 2000-2400° C. Solid SiC source material is evaporated and redeposited on a SiC seed substrate, which is controlled at a slightly lower temperature. Growth rates using the seeded sublimation technique are reported to range from 50-1000 μm/hr. Despite major development efforts over many years, several quality issues pertaining to the high temperatures required for boule formation remain.
Seeded sublimation techniques have several disadvantages: First, contamination from the graphite crucibles and source material lead to unacceptable impurity levels in the bulk crystals. These impurities lead to high unintentional carrier concentrations in the material. High resistivity wafers required for several applications remain extremely expensive. Additionally, high levels of defects in the crystal remain a concern. SiC boules produced by sublimation contain high dislocation densities, micropipes, mosaic structures, and stress fields, resulting from the chamber materials and the high temperatures required for growth. Inhomogeneous temperature distribution increases the probability of graphite inclusion or Si droplet formation, which can lead to micropipes and other defects Particles shedding from the graphite crucible have also been observed as a significant culprit for defects. Reduction of the defect density of boules grown using seeded sublimation techniques have been attempted using variations of chamber geometries, crucible materials, and processing conditions, see U.S. Pat. Nos. 6,534,026 and 6,562,130. Finally, the geometries associated with this technique make it extremely difficult to produce large diameter boules due to nonuniform temperatures in the boule and source material as the diameter of the sublimation system increases.
Another technique now being explored for the formation of bulk SiC is liquid phase epitaxy (LPE). In this process large diameter boules are pulled from highly purified molten material. Most bulk semiconductor materials, such as Si, are produced using this technique. Major advantages of this technique include polytype stability and low defect density, but the low solubility of carbon at reduced temperatures and the high silicon vapor pressure make this method difficult for growing silicon carbide boules. Also, silicon is very reactive at high temperatures and no suitable crucible material has been found. Producing SiC boules in this fashion would require temperatures of ˜3200° C. at a pressure of 105 bar. These factors have so far made LPE growth impractical: see D. H. Hofmann and M. H. Muller, “Prospects of the use of liquid phase techniques for the growth of bulk silicon carbide crystals”, Mat. Sci. and Eng. B, vol. 61-62, pp. 29-39, 1999.
In still another technique, single crystal SiC samples up to 2 mm thick have been grown with quality comparable to crystals grown by seed sublimation using high-temperature chemical vapor deposition (HTCVD), also referred to as hot-wall CVD. This method is provided by Kordina et al, U.S. Pat. No. 5,704,985 and Ellison et al., “High temperature CVD growth of SiC”, Mat. Sci. and Eng. B, vol. 61-62, pp. 113-120, 1999. In HTCVD, high-purity semiconductor grade gases are reacted to form the SiC. The reactions typically occur in graphite walled chambers where the walls and the seed crystal are generally maintained at temperatures >2000° C. At these temperatures, growth rates of 600 μm/hr have been achieved. HTCVD takes advantage of the availability of low-cost gas supplies which have much higher purity than the solid SiC sources used in sublimation. The reduction of impurities in the starting material will facilitate growth of SiC with reduced defect density and carrier concentration in the bulk. Also sublimation techniques are usually batch processes, which limit the potential length of the boule. Conversely in HTCVD the gas supply is essentially inexhaustible with the correct set-up, and the flowrates can be continuously tightly controlled, which makes the potential boule length only limited by the chamber design. However, many of the same problems associated with evaporation techniques will occur with HTCVD, such as high levels of impurities in the SiC from the graphite walls, and stress on the crystal formed due to the high growth temperatures. In addition, the reactions that occur at the walls of HTCVD reactors limit the diameter of uniform growth due to heat and mass transfer considerations.
The main polytypes, or crystal orientations, discussed for SiC boules are 3C, 4H, and 6H. 3C has a cubic structure, whereas 4H and 6H have hexagonal crystal structures. Sublimation and HTCVD techniques generally produce 4H and 6H polytypes, but not 3C, due to the high temperatures required for growing boules.
Traditional chemical vapor deposition (CVD) methods have been utilized to grow thin epitaxial layers of SiC on a variety of substrates. The majority of the SiC thin films deposited in industry are done by plasma enhanced CVD (PECVD), which are used for etch stop/diffusion barrier layers in integrated circuits. Generally these thin films are amorphous or polycrystalline, although some work has been carried out on the growth of single crystal thin films. In thermal CVD precursor gases are delivered to a heated substrate where reactions occur forming a growth layer. This technique has been used to produce single crystal 3C—SiC using SiH4, C3H8, and H2 precursors at a substrate temperature of 1400° C., see S. Nishino et al., “Production of large area single crystal wafers of cubic SiC for semiconductor devices”, Appl. Phys. Lett. 42 460 (1983). Although thermal CVD can produce single crystal layers, growth rates of SiC tend to be too low for the economic production of boules. Hot filament CVD (HFCVD) has also been shown to produce epitaxial layers of single crystal 3C—SiC at temperatures as low as 780° C., see Z. Zhiyong et al., “Epitaxial monocrystalline SiC films grown on Si by HFCVD at 780° C.”, Mat. Sci. and Eng. B, vol. 75, pp. 177-179, 2000. The substrate temperature for these depositions are in sharp contrast to sublimation and HTCVD techniques, which require substrate temperatures in excess of 2000° C. The prior art shows that the HFCVD technique can produce monocrystalline SiC in layers that are relatively thin, but the systems used for the growth of thin layers are not designed to deposit for the length of time required to grow boules. Also these systems will have contamination and growth time issues associated with the filaments at temperatures required to produce the growth rates needed for economic boule growth.
The advantages of SiC devices have yet to be extensively realized on a commercial level because of the technology for growing SiC boules used to produce substrates is relatively immature. Despite years of optimization efforts, the sublimation techniques used to produce most commercial SiC wafers continue to be plagued by high contamination and defect levels. Also, the adoption of SiC as a substrate material has been limited by the high cost and small diameters of the wafers currently available. Thus there is a need for new methods and systems to produce high-quality bulk SiC crystals. This and other needs are addressed by the present invention.